There is a continuing interest in the generation of colour hardcopy from electronically stored images, e.g. in the fields of colour proofing, and in the reproduction of computer graphics in the form of 35mm slides, overhead transparencies, etc. This is commonly achieved by using the electronic signals to modulate the output of a suitable exposing source, such as a laser or a light emitting diode. Because such devices are normally of very low power, they demand the use of highly sensitive imaging materials in order to achieve realistic scan times. Sensitivities of the order of 0.1 to 1.0 ergs/cm.sup.2 are required, such as are shown by the high speed silver halide materials and electrostatic systems. These materials show a number of disadvantages, such as high cost, complex wet processing, and darkroom requirements.
The present invention provides a different method of converting electronic data to colour hardcopy, wherein the electronic signals modulate the optical transmission properties of a liquid crystal array rather than the output of an exposing device. This allows the use of a continuous, high power light source, and hence the use of low sensitivity (typically 10.sup.3 - 10.sup.2 ergs/cm.sup.2) imaging materials. Such materials can be cheap, dry-processable and white light handleable.
The use of LCD's as programmable light shutters (also known as valves, or gates, or masks) is known and such devices have been used to generate images by allowing transmission of light through the LCD in the image areas only. Japanese Patent Application Publication No. 61-223726 discloses an overhead projector in which information is displayed by a transmission type liquid crystal unit controlled by a microcomputer or word processor. British Patent Applications GB 2144869A and 2069155A and Research Disclosure 27457 disclose apparatus in which a liquid crystal cell is arranged between a light source and photosensitive material and controlled to form a latent image which is recorded on the photosensitive material. The photosensitive materials used include (1) electrostatic media, (2) sensitive paper of the kind which responds to ultra-violet light, and (3) encapsulated monomer, photoinitiator and dye which releases dye in unexposed regions. The production of full-colour hardcopy is not disclosed.
Laser-Addressed Liquid-Crystal Light Modulators for Color Electronic Imaging with Mead Microencapsulated Paper, Sun Lu and Aharon Hochbaum, SID 87 Digest discloses a system which comprises at least three smectic liquid crystal light modules on which are written separation images by means of infra red radiation from a laser diode. Mead color paper, which is a silverless, dry color print paper based upon microencapsulated monomer, photoinitiator and dye, is exposed through each separation image in turn and processed to form the final image.
The present invention provides an alternative system of imaging low sensitivity materials utilising a liquid crystal device.